Activity level (AL), a core dimension of nearly every temperament theory, is a highly salient feature of child behavior that has been linked to various facets of child health and development. Although there is considerable evidence of genetic influences on individual differences in AL, there are many unanswered questions about this important temperament dimension. First, little is known about the role that genetic factors play in he development of AL. Second, short-term, situational changes in AL have rarely been considered from a behavioral genetic perspective. Third, there is ample evidence of a relation between AL and behavior problems, but the question of how the relation arises has not been examined. Finally, we do not know which genes are involved in AL. The goal of the proposed research is to use a multi-method approach to explore these unanswered questions in a sample of 300 toddler twin pairs. At 2 and 3 years of age, twins will wear mechanical motion recorders for 48-hours in the home and in laboratory activity episodes. Observational ratings of AL and parent ratings of AL and behavior problems will be obtained at each age. DNA will be obtained from cheek scrapings collected from twins and parents. The specific aims of this project are to: 1) Investigate the role of genetic influences on developmental change in AL; 2) Investigate genetic influences on situational change on AL within each age; 3) Determine the extent to which the agreement between mechanically-assessed AL and parent and observer ratings of AL is genetically mediated; 4) Examine genetic and environmental contributions to the relation between AL and behavior problems; and 5) Examine candidate gene associations with AL. The proposed longitudinal study of activity level will go beyond the basic nature-nurture question which asks whether genetic influence is important and will begin to address the etiological processes by which change in AL takes place The molecular genetic component to this research will complement the quantitative genetic analyses by providing the opportunity to identify specific genes for AL. Our multi-method, multi-situation approach will allow a more complete picture of child behavior, moreover the finding of similar results across methods should inform about the robustness of the results.